Data Exchanges

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Data Exchanges

MOV, PUSH, and POP opcodes all involve copying the data found in the source address to the destination address; the original data in the source is not changed. Exchange instruc­tions actually move data in two directions: from source to destination and from destination to source. All addressing modes except immediate may be used in the XCH (exchange) opcodes:

Mnemonic

Operation

XCH A,Rr

Exchange data bytes between register Rr and A

XCH A,add

Exchange data bytes between add and A

XCH A,@Rp

Exchange data bytes between A and address in Rp

XCHD A,@Rp

Exchange lower nibble between A and address in Rp

Exchanges between A and any port location copy the data on the port pins to A, while the data in A is copied to the port latch. Register A is used for so many instructions that the XCH opcode provides a very convenient way to "save" the contents of A without the necessity of using a PUSH opcode and then a POP opcode.

The following table shows examples of data moves using exchange opcodes:

Mnemonic

Operation

XCH A,R7

Exchange bytes between register A and register R7

XCH A,0F0h

Exchange bytes between register A and register B

XCH A,@R1

Exchange bytes between register A and address in R 1

XCHD A,@R1

Exchange lower nibble in A and the address in R 1

CAUTION

All exchanges are internal to the 8051.

All exchanges use register A.

When using XCHD, the upper nibble of A and the upper nibble of the address location in Rp do not change.

This section concludes the listing of the various data moving instructions; the remain­ing sections will concentrate on using these opcodes to write short programs.

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